Gait transitions in C. elegans
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The ability to switch between different forms of locomotion is critical to many aspects of survival, whether it is switching from walking to running to evade predators, or switching to a slower gait to obtain food. Uncovering the mechanisms behind gait transitions has implications for many fields, from treating Parkinson Disease to understanding the impact of drugs of abuse on movement. However, the mechanisms of gait transitions are not well understood. The experiments outlined in this thesis sought to understand the neuronal basis for gait switching. This work employed the nematode Caenorhabditis elegans, a unique model organism chosen for its genetic tractability and fully characterized nervous system. C. elegans displays different forms of motion: crawling on land and swimming in liquid. First, I sought to determine the mechanisms for switching between these forms of motion in collaboration with Dr. Andres Vidal-Gadea. In the process, we discovered that crawling and swimming actually represent distinct gaits in contrast to recent reports that suggested they were merely a single gait. We further elucidated mechanisms for gait transition in C. elegans. For instance, we found that the transition to crawling required viii the D1-like dopamine receptors DOP-1 and DOP-4; and activation of dopamine neurons via the light-activated cation channel Channelrhodopsin2 was sufficient to induce crawling behavior in worms immersed in liquid. Conversely, photoactivation of serotonergic neurons expressing Channelrhodopsin2 induced swim-like behavior on land. Finally, laser microablation of dopaminergic or serotonergic neurons was sufficient to impair the transition to crawl or swim, respectively. Together these results show that transitions to crawling and swimming are controlled by dopamine and serotonin respectively. Next I wanted to better understand how gait transitions are impaired by a drug of abuse, alcohol. I found that, as in other organisms, ethanol disrupts gait transitions, causing worms in water to inappropriately transition from swim to crawl and to display other land-specific behaviors. Animals lacking the D1-like dopamine receptor DOP-1 were resistant to the ethanol-induced transition to crawl. Finally, I found that several interneurons required for the transition to crawl. Specifically, laser microablation of the DOP-4 receptor-expressing neuron RID or the DOP-1-expressing neurons PQR or RIS resulted in a significant impairment in the time to crawl onset. Overall, the findings presented in this thesis represent the first evidence that C. elegans uses an evolutionarily conserved mechanism to transition between gaits and provides the beginning of a molecular description of gait transitions.